LECTURE 26
BME 4058: Medical Imaging
Course Instructor: Dr. Amritanshu Gupta
Department of BME
MIT (MAHE)
amritanshu.gupta@manipal.edu 1
Medical Resonance Imaging
Longitudinal Relaxation
MRI Machine Structure Net Magnetization Vector
(T1 Recovery)
Cartesian Planes for MRI Radiofrequency (RF) Pulse T1 Weighted Images
Nuclear Magnetic Signal Detection:
Resonance (NMR) Induction of Current T2 Weighted Images
Protons in Magnetic
Free Induction Decay (FID) k-Space
Fields
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Medical Resonance Imaging
❖ The MRI machine consists of several components, each
with a specific function in image creation.
❖ The most critical parts include multiple layers of
magnets.
MRI Machine Structure ❖ These magnets are essential for manipulating the
protons in the body and generating signals from them.
❖ Unlike other imaging methods such as X-ray or
ultrasound, MRI uses signals from the patient’s body,
specifically from hydrogen atoms. These signals are
generated and detected by the machine to create
images.
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Medical Resonance Imaging
MRI uses a three-dimensional Cartesian coordinate system to map
the body in different orientations.
❖ Z-axis (Longitudinal Axis): This axis runs from head to toe along
the length of the body. It is used to differentiate between slices
taken along the body’s length.
Cartesian Planes for MRI
❖ X-Y Plane (Transverse Plane): This is a cross-sectional plane that
cuts horizontally through the body, giving MRI the ability to
capture slice images of the internal organs and tissues.
Understanding this coordinate system is crucial for interpreting MRI
images because it helps identify where slices are taken and how
different sections of the body are visualized.
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Medical Resonance Imaging
Principle: MRI is based on the principle of nuclear magnetic resonance
(NMR). This technique involves manipulating atomic nuclei to produce
signals that can be used to create images.
The hydrogen atom is primarily used in MRI because:
i. Abundance: Hydrogen is the most abundant atom in the body,
Nuclear Magnetic especially in water and fat tissues.
Resonance (NMR)
ii. Non-zero spin: Hydrogen nuclei (protons) have a non-zero spin,
which allows them to behave like tiny bar magnets with a magnetic
moment.
The hydrogen proton has a magnetic moment, meaning it has both a
magnitude (strength) and direction (north and south poles).
In MRI, these protons act like small magnets that can be manipulated
by an external magnetic field.
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Medical Resonance Imaging
Alignment and Precession of Hydrogen Atoms:
When placed in an external magnetic field (e.g., in an MRI machine),
hydrogen protons can align either parallel or anti-parallel to the
magnetic field.
i. Parallel alignment: Protons align in the direction of the
magnetic field and are in a lower energy state.
Protons in Magnetic ii. Anti-parallel alignment: Protons align opposite to the magnetic
Fields field and are in a higher energy state.
Precession: In addition to aligning with the magnetic field, hydrogen
protons also spin or precess around their own axis, similar to a
spinning top. This precession occurs at a specific frequency called the
Larmor frequency.
42.58 MHz/T
The stronger the magnetic field (B₀), the faster the hydrogen protons will precess. This precession
is key for generating a measurable MRI signal.
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Medical Resonance Imaging
Alignment and Precession of Hydrogen Atoms:
When placed in an external magnetic field (e.g., in an MRI machine),
hydrogen protons can align either parallel or anti-parallel to the
magnetic field.
i. Parallel alignment: Protons align in the direction of the
magnetic field and are in a lower energy state.
Protons in Magnetic ii. Anti-parallel alignment: Protons align opposite to the magnetic
Fields field and are in a higher energy state.
Precession: In addition to aligning with the magnetic field, hydrogen
protons also spin or precess around their own axis, similar to a
spinning top. This precession occurs at a specific frequency called the
Larmor frequency.
42.58 MHz/T
The stronger the magnetic field (B₀), the faster the hydrogen protons will precess. This precession
is key for generating a measurable MRI signal.
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Medical Resonance Imaging
Alignment and Precession of Hydrogen Atoms:
In a large population of protons, more will align in the lower energy
(parallel) state than in the higher energy (anti-parallel) state.
The sum of all these individual magnetic moments creates a net
magnetization vector.
Net Magnetization Vector
Orientation: This net magnetization vector lies along the longitudinal
axis (Z-axis) when no external RF pulse is applied.
To Note: No Transverse Magnetization: In the absence of external manipulation, the precessing
protons are out of phase with each other in the transverse plane (X-Y plane), resulting in no
measurable transverse magnetization. Only the longitudinal component is present.
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Medical Resonance Imaging
Flipping the Net Magnetization Vector:
To generate a measurable signal, an RF pulse is applied at the Larmor
frequency of the hydrogen protons (as calculated using the Larmor
equation). This is called resonance.
Resonance: When the frequency of the RF pulse matches the precession
Radiofrequency (RF) Pulse frequency of the protons, energy is transferred efficiently, causing the
net magnetization vector to tilt away from the Z-axis (longitudinal
direction) and into the X-Y plane (transverse plane).
Flip Angle: The RF pulse "flips" the net magnetization vector by a
specific angle, often 90° for basic MRI sequences. The angle depends on
the duration and strength of the RF pulse.
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Medical Resonance Imaging
Faraday’s Law of Induction:
Once the net magnetization vector is flipped into the transverse plane, it
begins to precess around the Z-axis.
According to Faraday’s Law of Induction, a changing magnetic field
Signal Detection: induces an electric current in a nearby conductor.
Induction of Current
In MRI, this current is detected by receiver coils positioned around the
patient.
The Detected Signal: The precession of the magnetization vector in the
transverse plane induces a measurable current in the coils. This signal is
then used to create an MRI image.
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Medical Resonance Imaging
Decay of Transverse Magnetization:
After the RF pulse is turned off, the protons gradually lose phase
coherence due to slight differences in their precession frequencies.
Free Induction Decay (FID) This leads to a reduction in the net magnetization in the transverse
plane, a process known as free induction decay (FID) or T2 decay.
T2 Decay: The rate at which the signal decays is tissue-dependent. Some
tissues lose coherence more quickly than others, which helps
differentiate between them in MRI images.
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Medical Resonance Imaging
Regaining Longitudinal Magnetization:
After the RF pulse ends, the net magnetization vector slowly returns to
its original position along the Z-axis, a process called longitudinal
relaxation or T1 recovery.
Longitudinal Relaxation
(T1 Recovery) Different tissues recover longitudinal magnetization at different rates,
and this recovery contributes to image contrast.
To Note: The processes of T1 recovery and T2* decay happen simultaneously but are independent of
each other. The differences in these relaxation rates are exploited to create contrast in MRI images
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Medical Resonance Imaging
These images emphasize differences in T1 recovery times. Fat tissues
recover quickly and appear bright, while water (CSF) has a longer T1
recovery time and appears dark.
T1 Weighted Images
Adjustments in repetition time (TR) and echo time (TE) control how
much longitudinal magnetization has been regained, which affects
image contrast.
T2-weighted images emphasize differences in T2 decay times. Water
retains its signal longer and appears bright, while fat loses its signal faster
and appears darker.
T2 Weighted Images
TR and TE parameters are adjusted to capture differences in the loss of
transverse magnetization over time.
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Medical Resonance Imaging
Repetition time (TR) is the time between successive RF pulses in an MRI sequence. It controls how
much time is allowed for longitudinal magnetization to recover before the next RF pulse is applied.
✓ If TR is short, tissues with shorter T1 recovery times (like fat) will recover more of their
Repetition Time longitudinal magnetization, while tissues with longer T1 recovery times (like water) will not have
fully recovered. This creates a contrast where fat appears bright and water appears dark.
✓ If TR is long, most tissues will have more time to recover, reducing the contrast between fat and
water, because even water will have regained more of its longitudinal magnetization.
Echo time (TE) is the time between the application of the RF pulse and the point at which the
signal is measured (the echo is collected).It controls how much transverse magnetization has
decayed due to T2 relaxation (transverse relaxation), which affects the signal intensity.
Echo Time
For T1-weighted images, a short TE is used so that the image contrast is dominated by differences
in T1 recovery rather than T2 decay. A short TE minimizes the contribution of T2 relaxation,
allowing the image to reflect the T1 recovery properties of different tissues.
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Medical Resonance Imaging
k-Space is a grid where raw MRI data is stored. Each point in k-space represents a
specific spatial frequency of the final image.
Central k-Space contains low-frequency data, representing general image contrast, while Peripheral
k-Space contains high-frequency data that encodes fine details.
k-Space
Image Reconstruction : Data stored in k-space is converted into a spatial image using a mathematical
operation called the Fourier Transform.
The central part of k-space determines the overall brightness and contrast of the image, while the
edges contribute to the sharpness and detail.
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Medical Resonance Imaging
k-Space is a grid where raw MRI data is stored. Each point in k-space represents a
specific spatial frequency of the final image.
Central k-Space contains low-frequency data, representing general image contrast, while Peripheral
k-Space contains high-frequency data that encodes fine details.
k-Space
Image Reconstruction : Data stored in k-space is converted into a spatial image using a mathematical
operation called the Fourier Transform.
The central part of k-space determines the overall brightness and contrast of the image, while the
edges contribute to the sharpness and detail.
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Medical Resonance Imaging
https://www.youtube.com/watch?v=gtnOlotFgUY&list=PLWfaN
qiSdtzVp5u79H_sFE4IPcjgwhKdQ&index=1
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Thank You
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